High-energy resummation of direct photon production at hadronic colliders

Direct photon production is an important process at hadron colliders, being relevant both for precision measurement of the gluon density, and as background to Higgs and other new physics searches. Here we explore the implications of recently derived results for high-energy resummation of direct photon production for the interpretation of measurements at the Tevatron and the LHC. The effects of resummation are compared to various sources of theoretical uncertainties like PDFs and scale variations. We show how the high-energy resummation procedure stabilizes the logarithmic enhancement of the cross section at high energy which is present at any fixed order in the perturbative expansion starting at NNLO. The effects of high-energy resummation are found to be negligible at Tevatron, while they enhance the cross section by a few percent for pT?10 GeV$p_T?10\text{GeV}$ at the LHC. Our results imply that the discrepancy at small pT$p_T$ between fixed order NLO and Tevatron data cannot be explained by unresummed high-energy contributions.     

Effects of threshold resummation

We investigate effects of threshold resummation of logarithmic corrections lnN in Mellin space quantitatively. Threshold resummation leads to enhancement of next-to-leading-order QCD predictions for jet production at large jet transverse energy, which is in the trend indicated by experimental data. We show that this enhancement is completely determined by the behavior of threshold resummation at small N, the region where hierarchy among different powers of lnN is lost and current next-to-leading-logarithm resummation is not reliable. Our analysis indicates that more accurate threshold resummation formalism should be developed in order to obtain convincing predictions.     

Charm and bottom quark production cross sections near threshold

The cross sections for charm and bottom quark production in the threshold region are discussed. We consider the effects of an all order resummation of initial state soft-plus-virtual gluon radiation on the total cross sections compared to the order $←pha_{s}^{3}$ results.     

Nearside—farside theory of elastic angular scattering for strongly absorptive collisions

A systematic investigation has been made of ways for combining the method of Yennie, Ravenhall and Wilson (YRW) for the resummation of a partial wave Legendre series with the nearside—farside (NF) angular decompositions of Fuller and Hatchell. Of the resulting four procedures, the most useful one first applies the YRW resummation method m times (m = 0,1,2,…) to the partial wave scattering amplitude, followed by the Fuller NF decomposition. This NF procedure performs best for the physical interpretation of structure in the angular scattering of strongly absorptive elastic collisions. Numerical results from all four NF procedures are reported for a partial wave series possessing a simple parametrized scattering matrix element. The theory and calculations are relevant to atom—atom and atom—molecule elastic scattering in the presence of other open channels, for example, chemical reaction.     

QCD resummation for single spin asymmetries

We study the transverse momentum dependent factorization for single spin asymmetries in Drell-Yan and semi-inclusive deep inelastic scattering processes at one-loop order. The next-to-leading order hard factors are calculated in the Ji-Ma-Yuan factorization scheme. We further derive the QCD resummation formalisms for these observables following the Collins-Soper-Sterman method. The results are expressed in terms of the collinear correlation functions from initial and/or final state hadrons coupled with the Sudakov form factor containing all order soft-gluon resummation effects. The scheme-independent coefficients are calculated up to one-loop order.     

All-order results for soft and collinear gluons

Some general features and some recent developments concerning the resummation of long-distance singularities in QCD and in more general non-Abelian gauge theories are reviewed. The field-theoretical tools of the trade are emphasized, with the focus mostly on the exponentiation of infra-red and collinear divergences in amplitudes, which underlies the resummation of large logarithms in the corresponding cross-sections. Some recent results concerning the conformal limit, notably the case of N = 4 superymmetric Yang-Mills theory are also described.      

Non-abelian effects on wake potential in quark–gluon plasma

In this paper, based on the dielectric function calculated with the hard thermal loop (HTL) resummation approach, we study the wake potential induced by a fast parton traveling through quark–gluon plasma (QGP). In contrast to the wake potential calculated in the HTL approximation, our results show that non-abelian effects from the resummation calculation enhance the anisotropy of the wake potential. By investigating the relation between the anisotropic properties of the wake potential and the imaginary part of the dielectric function, we give some explanations for the enhancement of the anisotropy.     

Resummation for observables at TeV colliders

I review the status of, and discuss recent progress in the field of resummation, concentrating on QCD effects for hadron collider observables.     

Threshold resummation for gaugino pair production at hadron colliders

We present a complete analysis of threshold resummation effects on direct light and heavy gaugino pair production at the Tevatron and the LHC. Based on a new perturbative calculation at next-to-leading order of SUSY-QCD, which includes also squark mixing effects, we resum soft gluon radiation in the threshold region at leading and next-to-leading logarithmic accuracy, retaining at the same time the full SUSY-QCD corrections in the finite coefficient function. This allows us to correctly match the resummed to the perturbative cross section. Universal subleading logarithms are resummed in full matrix form. We find that threshold resummation slightly increases and considerably stabilizes the invariant mass spectra and total cross sections with respect to the next-to-leading order calculation. For future reference, we present total cross sections and their theoretical errors in tabular form for several commonly used SUSY benchmark points, gaugino pairs, and hadron collider energies.     

The dilepton-production cross section in principal value resummation

Using a recent calculation of the perturbative hard part for dilepton production that sums large threshold corrections to all orders in perturbative QCD, we compute the corresponding cross sections. The hard part has been evaluated using principal value resummation and contains all singular momentum-dependent corrections. We also include a resummation of large Sudakov terms, which are independent of parton momenta. We give predictions for the dilepton-mass distribution, the rapidity distribution and the rapidity-integrated K-factor at fixed-target energies and compare with various experimental results in several kinematic regimes. We find that principal value resummation produces cross sections that are finite and well-behaved. For both protons and anti-protons on fixed targets, the resummed cross sections are, in general, in excellent agreement with the data.     

Technical remarks and comments on the UV/IR-mixing problem of a noncommutative scalar quantum field theory

The possibility of a resummation procedure in order to cure the UV/IR-mixing problem of noncommutative field theories is discussed. The method is presented for a scalar φ⁴ theory on Euclidean space. Finally, we sketch the idea of resummation forU(1)-gauge theories.     

Resummed pQCD for <Emphasis Type="Italic">W</Emphasis><Superscript><Emphasis Type="Italic">±</Emphasis></Superscript> and <Emphasis Type="Italic">Z</Emphasis><Superscript>0</Superscript> transverse momentum spectra at RHIC and LHC

The transverse momentum distributions of W_± and Z₀ are predicted at RHIC and LHC. A resummation formalism with power corrections to the renormalization group equations is used. Shadowing effects are discussed and found to be unimportant at RHIC, but important for LHC. We study the enhancement of power corrections due to multiple scattering in nuclear collisions and numerically illustrate the weak effects of the dependence on the nuclear mass.     

Influence of long-range effects on the critical behavior of three-dimensional systems

The Padé-Borel resummation technique is used to describe field-theoretically, in the two-loop approximation, the behavior of Ising systems with long-range effects directly in a three-dimensional space. The renormalization-group equations are analyzed and the fixed points governing the critical behavior of the system are determined. It is shown that the long-range effects can bring about a change in both the regime of critical behavior and the kind of phase transition.     

Soft-gluon resummation for single-particle inclusive hadroproduction at high transverse momentum

We consider the cross section for one-particle inclusive production at high transverse momentum in hadronic collisions. We present the all-order resummation formula that controls the logarithmically-enhanced perturbative QCD contributions to the partonic cross section in the threshold region, at fixed rapidity of the observed parton (hadron). The explicit resummation up to next-to-leading logarithmic accuracy is supplemented with the computation of the general structure of the near-threshold contributions to the next-to-leading order cross section. This next-to-leading order computation allows us to extract the one-loop hard-virtual amplitude that enters into the resummation formula. This is a necessary ingredient to explicitly extend the soft-gluon resummation beyond the next-to-leading logarithmic accuracy. These results equally apply to both spin-unpolarized and spin-polarized scattering processes.     

Lowx physics and structure functions

In this talk I review the behaviour of structure functions at low values of Bjorkenx and discuss the theoretical underpinnings with particular attention to resummation schemes. I present the need for ‘less inclusive’ events to distinguish between various resummation schemes and discuss the various difficulties in differentiating experimentally between different schemes.     

Noisy nonlinear coupled equations—Some new insights

We discuss the use of coupled nonlinear stochastic differential equations to model the dynamics of complex systems, and present some analytical insights into their critical behaviour. These concern in particular the role of infrared divergences which show up in a self-consistent resummation of perturbation theory (mode-coupling approximation), and their effects on critical exponents obtained in earlier work.     

QCD versus recent experimental data on lepton pair production at accelerator energies

New and precise experimental results on Drell-Yan pair production at accelerator energies are now available, both in π^? andp-nucleus interactions. These data are analyzed within the QCD framework including soft gluon resummation. It is shown that a quantitative agreement of the theory with experimental data can be reasonably managed. In particular, the soft gluon resummation is shown to build up nicely the so-calledK-factor. The pion formfactor is deduced from experimental data, but is rather sensitive to the nuclear structure functions we use. Definite nuclear structure functions are needed.     

Differential production cross section of Z bosons as a function of transverse momentum at sqrt

We present a measurement of the transverse momentum distribution of Z bosons produced in p&pmacr; collisions at sqrt[s] = 1.8 TeV from data collected by the DO experiment at the Fermilab Tevatron Collider. We find good agreement between our results and current resummation calculations, and also use our data to extract nonperturbative parameters for a particular version of the resummation formalism. The resulting values are significantly more precise than obtained in previous determinations.     

Parity-violating spin asymmetries in polarized proton-proton scattering with hadronic final states

Longitudinal, parity-violating single spin asymmetries in proton-proton collisions for RHIC kinematics are considered. The focus of this study is on the production of a single jet as well as a hadron containing a charm quark. While the asymmetry for jets is rather small, considerably larger effects can be expected for the production of charm because of the strongly reduced QCD background. The general outcome of a fixed order calculation is not much affected by the resummation of large threshold logarithms.